The present invention relates to a method of thermal treatment of continuously moving metal bands in particular aluminum bands, which is subjected along its treatment path to hot treatment gas and is floatingly guided.
The present invention also relates to an arrangement for thermal treatment of continuously moving bands which has a treatment chamber with a system of blowing boxes for guiding the metal bands in a floating condition by means of a hot treatment gas.
Methods and arrangements of the above mentioned general type are disclosed for example in the German document DE-PS 33 18 861 and in the European patent document EP-B1-0 192 169.
During the thermal treatment of continuously moving aluminum bands with a width of more than 0.5-1 m, there is a problem in known methods that with increasing band width there is an increasing moving speed they are distorted: in particular the edge regions of the bands assume a wavy shape. This waviness acts in disadvantageous manner during the later processing.
The cause of this waviness is probably the heat expansion of the band. Due to the heat expansion the band edges assume a curved shape in the heating region. A surface element located in the edge region and having a rectangular shape in stationary condition is deformed in the heating phase to a acute-angle parallelogram. With the utilized temperature the elasticity limit can be exceeded, so that the plastic or in other words permanent deformation occurs which leads to the expansion in the edge region. This expansion leads in turn to formation of the waves. During the subsequent cooling the reverse situation takes place.
It is possible that the edge expansions can be also partially explained by the fact that the insignificant lateral deviations from the straight movement direction in the edge regions lead to stresses with which in condition of the increased temperature plastic deformations occur.
For preventing the undesirable edge deformation it wa proposed on the one hand to supply the heat over the whole width as uniformly as possible, and on the other hand, to guide the band exactly. These efforts do not always bring the desired results. It has been shown that especially in the case of high moving speeds the waviness cannot be avoided. Therefore it was necessary to reduce the through output to maintain the waviness within acceptable limits.